Work around inotify limits with Core CLR on Linux

- #3066
- cache `HostingEnvironment` instances per application root
  - avoid creating a system-limited `FileSystemWatcher` per test

Author: dougbu

test/Microsoft.AspNet.Mvc.FunctionalTests/TestHelper.cs

@@ -2,14 +2,15 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Concurrent;
 using System.IO;
 using System.Reflection;
 using Microsoft.AspNet.Builder;
 using Microsoft.AspNet.Hosting;
 using Microsoft.AspNet.TestHost;
-using Microsoft.Framework.DependencyInjection;
 using Microsoft.Dnx.Runtime;
 using Microsoft.Dnx.Runtime.Infrastructure;
+using Microsoft.Framework.DependencyInjection;
 
 namespace Microsoft.AspNet.Mvc.FunctionalTests
 {
@@ -18,6 +19,13 @@ public static class TestHelper
         // Path from Mvc\\test\\Microsoft.AspNet.Mvc.FunctionalTests
         private static readonly string WebsitesDirectoryPath = Path.Combine("..", "WebSites");
 
+        // Minimize PhysicalFileProviders allocated. Otherwise tests create far more than would ever be needed in
+        // production. On some platforms this aberration causes IOExceptions and irrelevant test failures. One example
+        // is Core CLR on Linux, where the functional tests exceed the default fs.inotify.max_user_instances and
+        // fs.inotify.max_user_watches.
+        private static readonly ConcurrentDictionary<string, HostingEnvironment> HostingEnvironments =
+            new ConcurrentDictionary<string, HostingEnvironment>(StringComparer.Ordinal);
+
         public static TestServer CreateServer(Action<IApplicationBuilder> builder, string applicationWebSiteName)
         {
             return CreateServer(builder, applicationWebSiteName, applicationPath: null);
@@ -114,8 +122,8 @@ private static void AddTestServices(
                 applicationBasePath,
                 applicationWebSiteName);
             services.AddInstance<IApplicationEnvironment>(environment);
-            var hostingEnvironment = new HostingEnvironment();
-            hostingEnvironment.Initialize(applicationBasePath, environmentName: null);
+
+            var hostingEnvironment = HostingEnvironments.GetOrAdd(applicationBasePath, CreateHostingEnvironment);
             services.AddInstance<IHostingEnvironment>(hostingEnvironment);
 
             // Injecting a custom assembly provider. Overrides AddMvc() because that uses TryAdd().
@@ -139,6 +147,14 @@ private static string CalculateApplicationBasePath(
                 Path.Combine(appEnvironment.ApplicationBasePath, websitePath, applicationWebSiteName));
         }
 
+        private static HostingEnvironment CreateHostingEnvironment(string applicationBasePath)
+        {
+            var hostingEnvironment = new HostingEnvironment();
+            hostingEnvironment.Initialize(applicationBasePath, environmentName: null);
+
+            return hostingEnvironment;
+        }
+
         private static IAssemblyProvider CreateAssemblyProvider(string siteName)
         {
             // Creates a service type that will limit MVC to only the controllers in the test site.